Stainless-steel nut for cabochon mandrel.
This nut can be used to adjust the length of the free thread on the mandrel. The nut is screwed between the cabochon mandrel and the cabochon disc.

For example: if one nut is screwed on between the mandrel and the disc, only 2 mm remain on the front of the mandrel (for mandrels with a 6 mm thread length). With two nuts, the thread at the front of the chuck will not be visible.

Video tutarial available here.
Photo tutorial available in German here.